1. Field of the Invention
The present application relates to the field of transmission systems. More particularly, embodiments within the scope of the application and claims relate to methods, systems, sub-systems, assemblies, and components for providing constant engagement during power transmission, and during changes of gear ratios in very small, and possibly infinitely small, increments.
2. Related Technology
From nearly the beginning of mechanical engines, the purpose and design of an engine has been focused, to at least some degree, on allowing a small engine to move a large load. As engines evolved and technology became more sophisticated, engines were developed having transmissions with multiple ratios to allow the engine to start moving the load with a low ratio and to incrementally step up to higher ratios as the load began moving. In this manner, a transmission can make more effective use of the engine's torque and keep the engine operating near an appropriate speed. Moreover, an engine can operate within a narrow range of speeds while providing a wider range of output speeds.
To effect an incremental change in gear ratio, a manual transmission uses various separate driven gears of different sizes in connection with one or more drive gears. As a gear ratio change is made, a drive gear disengages from the driven gear and re-engages with a different gear. For example, a clutch may disengage a drive gear from a driven gear and then re-engage the same or a different drive gear with a second driven gear having a different radius. As the newly engaged gears have different radii—or levers—the gear ratio is changed. To effect this gear ratio change, however, the drive gear must be temporarily disconnected from all driven gears, such that the power source is also temporarily disconnected from the load while the gear ratio change is made.
Automatic transmissions also make incremental changes in gear ratio by disconnecting the engine from the load. To do so, automatic transmissions typically use one or more planetary gear sets which are used in connection with a series of clutches and bands that are driven by hydraulic system. To change between gear ratios, valves within the hydraulic system are used to control hydraulic pressure which activates various clutches and bands so as to connect and disconnect the carriers and various gears of the automatic transmission from the engine. Based on the specific clutches and bands that engage and disengage, the transmission achieves a predetermined gear ratio change.
When the power source is disconnected or disengaged from the load, the engine coasts until the power source is reconnected to the load. As the engine coasts, however, a moving load begins to lose momentum. For instance, a semi-tractor trailer or other moving vehicle may be moving uphill when a gear change is required. By pushing in the clutch or otherwise disconnecting the power source from the load, the engine RPMs are decreased, turbos may be dumped, and torque can be lost. As a result, the driver often must shift two or three gears down because re-engaging the power source will not occur fast enough to maintain the engine RPMs at a drop of only one or two gears down. This results in an inefficient use of the engine horsepower and fuel.
Similarly, where a tractor is pulling a load such as a plow, disconnecting the engine from the load reduces the momentum of the tractor and the plow. While the tractor may be able to coast, the plow is less likely to coast. For example, when the plow loses momentum it may catch on the ground being plowed and thereby drag against and stop the tractor from coasting. The plow may catch and stop with a sudden movement that can damage the tractor and potentially injure the operator. Therefore, to avoid damage and injury, the tractor operator may drive the tractor and plow in a low gear to avoid the need to shift gears although a higher gear would allow the tractor to more quickly plow the field, consume fuel more efficiently, and make use of the momentum to obtain a draft of the plow.
In addition, many other applications have previously been unable to take advantage of the benefits of a variable speed transmission because disconnection of the power source from the load makes the application unsafe or impractical. For example, an elevator could benefit from gear ratio changes to change the speed of its ascent or descent. However, disconnecting the power source during ascent or descent would cause the elevator carriage to coast and could make the variable speed transmission unsafe for the elevator passengers.
A conveyor system such as those used in manufacturing or mining operations could also benefit from variable speeds. For example, as the system starts up, the conveyor belt could be started at a slow speed and the speed then increased for full operation. Many conveyor belts are, however, loaded with material and/or are miles long, thereby creating a large load that must be moved. If a gear ratio change is made by even temporarily disconnecting the power source, the material and conveyor belt lose momentum and can prevent an efficient gear ratio change. As a consequence, materials often have to be removed from the belt just to get the conveyor moving and/or the conveyor system must be operated at a constant speed.
While variable speed transmissions provide numerous benefits, the problems of the disconnection of the power source from the load has caused engine and transmission designers to search for methods and systems that minimize the time the power source is disconnected and a drive gear is disengaged. To at least some degree, automatic transmissions have improved this time by automating the shifting between gears and changing gear ratios, but the change has not been fundamental, although such automatic transmissions have at least reduced the time between disconnecting and reconnecting the power supply. However, even automatic transmissions disconnect the engine from the drive gears, thereby causing a loss in torque for a time and failing to make an efficient use of the horsepower. Moreover, by operating with only a small group of discrete gear ratios—each having only one or a very few speeds at which the engine operates at optimum efficiency—the engine operates mostly in an inefficient range. Consequently, the engine must be capable of providing more horsepower, and must thus be larger, than would otherwise be required if an engine was more frequently running at an efficient speed. The inefficient use of these engines, in turn, burns more fuel than would an engine run at more efficient speeds.
In low torque applications, the problems associated with disconnecting the power source from the load have been reduced, to some extent, by continuously variable transmissions (CVT). A CVT typically uses two pulleys which are connected by a belt. The pulleys can include two oppositely oriented cones that face each other and which can be pulled together or pushed further apart by hydraulic pressure, centrifugal force, or spring tension. As one pulley is moved to position the belt over a larger radius portion, the other pulley is moved to position the belt over a smaller radius to keep the belt tight. As the position of the belt changes to engage portions of pulleys with differing widths, various gear ratios can be created. A similar concept that may also be considered a CVT also makes use of similar, complementary pulleys and cones. Instead of a belt, however, the CVT uses a rolling member that is sandwiched between the cones.
Regardless of whether a belt or a rolling member is used, however, the CVT system generally relies on friction to facilitate adjustment of gear ratios and provide power output. Friction introduces heat into the system, however, and as a result the wrapping member and rolling members heat up and are susceptible to wear damage, thereby requiring that the user repair or replace the parts. To reduce the frequency of repair, the frictional wrapping or rolling members can be toughened, such as through the use of a thicker belt or impregnation of the belt with metals or other tougher materials. However, as the belt strength is increased, the part costs increase. Moreover, sufficiently tough materials can cause the cones or pulleys within the transmission to wear and fail.
Moreover, because these systems are friction-based, they are typically only suitable for low torque applications, as high torque applications could cause excessive heating within the transmission, thereby causing even greater wear and failure of the transmission components. As a result, CVT transmissions are not considered scalable for a wide variety of low and high torque applications. In fact, the application of torque to a CVT in a high torque or high horsepower system may cause near immediate failure as the rolling member or wrapping member can melt or otherwise deteriorate due to the friction-induced heat.
Because the CVT systems have been seen as unacceptable alternatives in high-torque applications, additional efforts have been made within high-torque applications in an attempt provide little to no time gap between disconnection and reconnection of the power source and load. For example, John Deere produces tractors with a PowerShift transmission that uses a complex design to automatically do the clutching and disconnect a clutch and reconnect the clutch at the same time such that there is no real time gap and little to no torque loss. The transmission is, however, much larger than a standard transmission, and can house a large number of hydraulic lines inside the transmission. As a result, maintenance of the lines may be difficult, and the size of the engine further increases the size of the equipment and the weight or load that must be carried. Moreover, because of the complexity and size of the transmission, it can be cost prohibitive for certain applications, and it is not scalable for low torque or smaller applications.
Accordingly, a need exists for an improved transmission which is scalable and which can move between multiple gear ratios without disconnecting the power source from the load.